Hey there, given you're growing in soil, i would start the flush early, so about 2 weeks prior to harvest. That's because soil keeps much more of the excess nutrients stored than the other mediums like coco or peat for which a week is sufficient. (Pure hydro is easiest with 2 or 3 days, as you're exchanging the whole growing media with a reservoir change.)
Some people just flush with pure water, others flush with a lower amount of nutrients, which is what i would personally recommend when it comes to soil.
Reason is that you don't want to starve the plant fully, when it needs the most nutrients. Then again you also wouldn't want to give her too many nutrients when you're actually trying to remove the nutrient-buildup in the growing medium (and want the plant to metabolize what she already has inside her).
Depending on your water (if using tap or RO) I would start out with around an EC of 0.4 (with RO) and then with the next waterings move downwards.
Additional products that break up salts, like florakleen or AN's flawless finish can also help with removing the salt buildup from the growing medium, but obviously shouldn't be used together with nutrient-solution (unless it's "ripening" or a similar product that's meant to be made together with a flushing agent).
When it comes to the "when" i wouldn't look at the pistils (the hairs), but actually at the trichomes themselves..